1. Field of the Invention
The present invention relates to a method of aligning, with a sample measurement position, a probe of a probe microscope for measuring shape information such as surface roughness or a step of a sample surface, or physical information such as a dielectric constant or viscoelasticity, and a probe microscope operated by the same.
2. Description of the Related Art
In recent years, a probe microscope such as an atomic force microscope (AFM) having atomic resolution has been expected for shape measurement to evaluate a fine shape. The atomic force microscope which is a type of probe microscope has been expected as means for observing a surface shape of a novel insulating material, and investigations thereof have been conducted since the atomic force microscope was devised by G. Binnig (inventor of scanning tunneling microscope (STM)), et al.
An example of a schematic system structure of the probe microscope is described with reference to FIG. 6.
A sample 51 which is an object to be measured is placed on a fine movement mechanism 52 for three-dimensionally moving the sample. The fine movement mechanism 52 includes piezoelectric elements deformed in response to applied voltages, and finely aligns the sample 51 relative to a probe 53 opposed to the sample. The probe 53 is provided at a tip end of a beam member supported at only one end, which is called a cantilever 54.
FIG. 7 illustrates an example of a normal shape of the cantilever. A cantilever board 64 is provided with the cantilever 54 which is the beam member supported at only one end. The fine probe 53 is formed at the tip end of the cantilever 54 and has mainly a square pyramid shape with a height of 1 μm to 2 μm. The cantilever board 64, the cantilever 54, and the probe 53 are made of silicon or a silicon-based material and integrally formed using, for example, an anisotropic etching technique.
The cantilever board 64 including the cantilever 54 is held by a cantilever holder 55.
The fine movement mechanism 52 is located on a rough movement mechanism 56 including a stage for three-dimensionally moving the sample 51 and the probe 53. The rough movement mechanism 56 is a screw feed mechanism driven by a stepping motor.
A displacement detection system 57 for detecting a deformation of the cantilever based on a physical force such as an interatomic force, which the probe 53 receives from the surface of the sample, is provided on the cantilever 54 side. The displacement detection system 57 includes a semiconductor laser 58 for irradiating a rear surface of the cantilever 54 with light and a four-part photo detector 59 for detecting reflected light, and is a system called an optical lever detection system for detecting a displacement (distortion deformation) of the cantilever based on the fact that a position of the light incident on the photo detector is changed by the displacement of the cantilever 54.
A signal from the photo detector 59 is sent through an amplifier 60 to a Z-axis control feedback circuit 61 for controlling a Z-axis (vertical direction) interval between the sample 51 and the probe 53 to operate the fine movement mechanism 52, thereby controlling a Z-axis positional relationship between the probe 53 and the sample 51. In-plane scanning between the probe 53 and the sample 51 is performed by scanning with the fine movement mechanism based on a signal from an XY-driver circuit 62. The Z-axis control and the XY-driving are performed by a computer and a control system 63.
The probe 53 provided at the tip end of the cantilever 54 is brought close to the surface of the sample 51 by the rough movement mechanism 56. A deformation of the cantilever 54 resulting from a physical force such as an interatomic force, a magnetic force, or viscoelasticity, which the probe 53 receives from the surface of the sample 51, is detected by the displacement detection system 57. When the deformation becomes a predetermined deformation, it is determined that the probe 53 is aligned to a measurement region. Then, the rough movement mechanism 56 is stopped, and the Z-shaft of the fine movement mechanism 52 for relatively moving the sample 51 and the probe 53, which is located on the sample side or the cantilever side, is controlled to maintain an interval between the probe 53 and the surface of the sample 51. While the Z-shaft of the fine movement mechanism 52 is controlled, the rough movement mechanism 56 is driven to adjust a displacement amount of the Z-shaft of the fine movement mechanism 52, thereby aligning the probe 53 with the surface of the sample 51. The control is performed such that a deformation value of the cantilever 54 is maintained constant. The surface of the sample is measured during scanning using the fine movement mechanism 52, thereby visually imaging an in-plane shape of the sample and physical properties thereof.
An optical microscope is normally used as means for designating a location to be measured using the probe microscope, and there are a method of observing a region between the cantilever and the sample in an oblique direction and a method of performing an overhead observation using an optical member such as a prism (see, for example, JP 3023686 B). In the case of the oblique observation, a depth state on an opposite side of the optical microscope is uncertain, and hence alignment precision is low. On the other hand, in the case of the overhead observation, the alignment is easy. In general, in the case of the overhead observation, the focus of the optical microscope is adjusted on the rear surface opposed to the surface of the cantilever, to which the probe is attached, thereby verifying a probe position. Then, the focus of the optical microscope is adjusted on the surface of the sample based on the verified position, thereby determining a position to be observed using the probe microscope. As illustrated in FIG. 8A, assume that an optical microscope 71, a probe 72, and an observation object 74 of a sample 73 are aligned with an optical path 75 and a moving path 76 of an alignment unit (rough movement mechanism) for aligning the sample and the probe with each other. In such a case, the probe 72 can be aligned with the observation object 74 at a close position 77 between the probe 72 and the sample 73 without any problems, and hence the observation object to be observed using the optical microscope 71 can be observed using the probe microscope. However, unless adjustment is performed, an apparatus structural displacement occurs because the optical microscope and the rough movement mechanism are different members. As illustrated in FIG. 8B, even in the case of the overhead observation, an error 78 occurs, and alignment precision may be low. The error may vary at a magnification related to a geometric relationship as an interval between the focal position of the optical microscope and the close position 77 becomes larger. When the probe is brought close to the sample to perform alignment, the error may become smaller. However, in this case, the optical microscope observes the rear surface of the cantilever, and hence there is no difference between the focal position for observing the cantilever and the focal position for observing the sample. Therefore, when the surface of the sample is to be observed, the shadow of the cantilever interferes therewith, whereby the surface of the sample cannot be observed.
A scanning region of the probe microscope is normally several tens micrometers in size. When the alignment precision is low, the position may be outside the scanning region of the probe microscope. In this case, the method is conceivable, in which the scanning region of the probe microscope is increased in size. However, in a case where a smaller object is to be observed, when the scanning region of the probe microscope is large in size, the resolution is reduced because a data acquisition interval is large. Therefore, it takes time to find the observation object.
An example of a combination with the optical microscope is a method of adjusting the sample observation position and the probe position using a moving mechanism such as a stage based on a known sample for each cantilever exchange in a case where the optical microscope is provided at a position different from that of the probe (see, for example, JP 2909829 B and JP 06-201372 A). There is also a method of correcting the probe position using the optical microscope provided at a position separate from that of the probe position of the probe microscope (see, for example, JP 09-203740 A and JP 04-058102 A). Such a method may have alignment precision. However, an apparatus becomes larger, and hence the method is not suitable for a small apparatus.
Therefore, a probe microscope capable of easily aligning a position to be observed has been desired.